1. Field of the Invention
The present invention relates to an inverter circuit for use with a discharge tube or lamp such as a cold-cathode fluorescent lamp, a hot-cathode fluorescent lamp, a mercury arc lamp, a metal halide lamp, a neon lamp or the like.
2. Prior Art
It is necessary for turning on such a discharge tube or lamp to use a commercial AC power supply or a high-voltage power supply utilizing a commercial AC power supply and a lighting or starting circuit comprising a ballast for limiting a current. Recently, for the sake of miniaturizing such starting circuits and also of popularizing portable type devices or apparatus such as small size liquid crystal display devices each utilizing a discharge lamp, for example, as a back lighting source, various inverter circuits have been used for obtaining a high-voltage power supply from a low-voltage DC power source thereby turning on a discharge tube.
A capacitive ballast or an inductive ballast may be used in such an inverter circuit for a discharge tube, and a capacitive capacitor has generally been used as a ballast.
While an inverter circuit for a discharge tube of smaller size is required because of making portable equipments small in size and light in weight, it is known in general that peripheral components or parts such as a step-up transformer, capacitor and the like can be miniaturized by making a driving frequency for an inverter circuit higher so that the whole of an inverter circuit can be miniaturized in size.
However, as the driving frequency becomes higher, the influence of a parasitic or stray capacitance caused by a secondary winding of a step-up transformer, wiring or the like cannot be ignored.
In addition, in many cases, a closed magnetic flux type core, namely, an EI type core consisting of two magnetic pieces of E and I shapes or an EE type core consisting of two magnetic pieces of E and E shapes has been adopted as that of a step-up transformer used in an inverter circuit for a discharge tube on the basis of fundamental circuit design or plan in which a leakage of magnetic flux is considered to be harmful in efficiency.
It is a step-up transformer that occupies the largest space in an inverter circuit for a discharge tube, and the difficulty of miniaturization in size of the step-up transformer makes it impossible to miniaturize the whole inverter circuit in size.
Hence in order to reduce the step-up transformer in size, as the driving frequency for the inverter circuit for a discharge tube is made higher, a parasitic or stray capacitance or capacitances caused in a secondary winding of a step-up transformer, wiring and the like can gradually increase thereby affecting the operation of the inverter circuit, and thus it has a limitation to make the driving frequency higher.
Specifically, in a collector resonance type inverter circuit for a discharge tube as shown in FIG. 3, a ballast capacitor 22 is connected between one end of a secondary winding SW1 of a step-up transformer 21 and one electrode of a fluorescent lamp 24, and such ballast capacitor 22 normally has a capacitance of several picofarads (pF) to several tens picofarads though the capacitance thereof can differ depending on a driving frequency for the inverter circuit.
Whereas a parasitic capacitance 23 caused in the secondary side of the step-up transformer 21 and a parasitic capacitance 25 caused in the circumference of the fluorescent lamp 24 are normally of several picofarads, respectively.
The parasitic capacitance 25 can be increased in case a connecting wire between the secondary output of the step-up transformer 21 and the fluorescent lamp 24 is long, and hence there is a limitation on the length of the connecting wire, too.
In the above-mentioned inverter circuit, a high voltage induced across the secondary winding SW1 of the step-up transformer 21 is divided in voltage by a series combination of the ballast capacitor 22 and the parasitic capacitance 25 and this divided high voltage lower than the high voltage across the secondary winding SW1 is supplied to the fluorescent lamp 24.
Since, in circuit design, the ballast capacitor 22 is smaller in its capacitance as the driving frequency for the inverter circuit is higher, the ratio of the parasitic capacitance 25 to the ballast capacitor 22 becomes greater in the range in which the driving frequency is high. This causes the results that a voltage for discharge supplied to the fluorescent lamp 24 is lowered, which in turn causes the brightness or luminance of the fluorescent lamp 24 to decrease, and therefore there is needed such a consideration that turn ratio of the step-up transformer 21 is made greater than that determined by the circuit design or the like.
Moreover, a load as seen from the primary side is capacitive due to the influence of the ballast capacitor 22 and the parasitic capacitances 23, 25 and deteriorates the power factor.
This results in increase of a reactive current flowing through a collector winding (primary winding PW1 of the step-up transformer 21 one end of which is connected to the collector of a first transistor TR1 and the other end of which is connected to the collector of a second transistor TR2) and hence a copper loss or ohmic loss of the collector winding is increased thereby lowering the efficiency of the circuit. In FIG. 3, the step-up transformer is composed of a core 11, the primary winding PW1, a base winding PW2, and the secondary winding SW1, and IN1 and IN2 are input terminals of the inverter circuit to which a DC voltage is applied, C1 connected between the input terminals IN1 and IN2 is a capacitor for storing charges, CH1 is a choke coil for limiting a current, R1 and R2 are resistors connected to bases of the transistors TR1 and TR2, respectively, and C2 is a capacitor connected across the primary winding PW1.
For that reason, it is necessary to make it possible to use higher driving frequencies thereby further reducing the step-up transformer in size by working out a new circuit design inclusive of parasitic capacitances.
Also, a high voltage-resistant capacitor used as a ballast capacitor is requested to have high reliability, but there often occurs a failure or defect of an inverter circuit for a discharge tube due to a failure or defect of the high voltage-resistant capacitor. Hence it is desirable not to use a capacitor as a ballast from an aspect of the reliability.
In addition, it is possible to use an inductive choke coil as a ballast. However, in case of using an inductive load, there can occur that starting or continuation of oscillation of a self-excited inverter circuit for a discharge tube is difficult.
In order to resolve that problem, in an inverter circuit for a discharge tube using an inductive ballast, a capacitive load is added to the secondary side of the inverter circuit so as to cancel an inductive load as seen from the primary side thereof so that starting or continuation of oscillation of the inverter circuit can be easily done.
As described above, a step-up transformer used in a prior inverter circuit for a discharge tube has an EI type core consisting of two magnetic pieces of E and I shapes or an EE type core consisting of two magnetic pieces of E and E shapes adopted as a magnetic core thereof. The volume of the core of such shape occupies a considerable space in the whole inverter circuit, that is, it is the core of the step-up transformer that occupies a large space in the inverter circuit, and so the core is an obstacle or bar to miniaturization of the inverter circuit. Therefore, as long as a closed magnetic flux type step-up transformer is used in an inverter circuit, there is a limitation on miniaturization of the step-up transformer.
Accordingly, it is needed to implement the miniaturization of the step-up transformer by reconsidering or reviewing the shape of the core and the magnetic circuit.